Hi. I was looking for netbooks because i wanted to have a portable computer for learning pentesting with long battery life and I was concerned about how weak netbooks are and I wanted to ask what should i choose for about 250 - 300$BTW it needs to have no more than 12 inches.

While I'm not up to date on the current hardware I know this: my netbook works great for everything except brute force and wifi packet injection, I've had some problems with drivers in the past. If you want to do some wifi cracking, be sure the hardware is well supported under Linux or use external hardware, this also boosts reception. Battery life usually is not that much of a problem, worst case scenario you can get a larger one which at least doubles the time. My Asus Eee pc a few years back crashed after half a year, so I would advice against that (or maybe that was just back luck). After getting another brand, I haven't looked back and the thing still runs great, albeit showing its age a bit.